1-10
The high-order four bits of a multicast IPv4 address are 1110, indicating that this address is a multicast
address, and only 23 bits of the remaining 28 bits are mapped to a MAC address, so five bits of the
multicast IPv4 address are lost. As a result, 32 multicast IPv4 addresses map to the same MAC address.
Therefore, in Layer 2 multicast forwarding, a device may receive some multicast data addressed for
other IPv4 multicast groups, and such redundant data needs to be filtered by the upper layer.
2) IPv6 multicast MAC addresses
The high-order 16 bits of an IPv6 multicast MAC address are 0x3333, and the low-order 32 bits are the
low-order 32 bits of a multicast IPv6 address.
Figure 1-7 shows an example of mapping an IPv6
multicast address, FF1E::F30E:101, to a MAC address.
Figure 1-7 An example of IPv6-to-MAC address mapping
Multicast Protocols
z Generally, we refer to IP multicast working at the network layer as Layer 3 multicast and the
corresponding multicast protocols as Layer 3 multicast protocols, which include IGMP/MLD,
PIM/IPv6 PIM, MSDP, and MBGP/IPv6 MBGP; we refer to IP multicast working at the data link
layer as Layer 2 multicast and the corresponding multicast protocols as Layer 2 multicast protocols,
which include IGMP Snooping/MLD Snooping, and multicast VLAN/IPv6 multicast VLAN.
z IGMP Snooping, IGMP, multicast VLAN, PIM, MSDP, and MBGP are for IPv4, MLD Snooping,
MLD, IPv6 multicast VLAN, IPv6 PIM, and IPv6 MBGP are for IPv6.
z This section provides only general descriptions about applications and functions of the Layer 2 and
Layer 3 multicast protocols in a network.
Layer 3 multicast protocols
Layer 3 multicast protocols include multicast group management protocols and multicast routing
protocols.
Figure 1-8 describes where these multicast protocols are in a network.
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